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Two new versions of a computer virus that affects mobile phones were discovered in late December with new features that allow them to spread more quickly between vulnerable devices, according to antivirus company F-Secure Corp.

Cabir.H and Cabir.I are the latest versions of a worm that was first identified in June and affect Symbian Ltd. mobile phones. There are no reported infections from the new worms. However, F-Secure said that the new viruses fix a problem with earlier versions of Cabir that prevented that worm from spreading quickly between mobile phones.

Like the original Cabir worm, dubbed Cabir-A, the new Cabir variants spread between mobile phones using a specially formatted Symbian operating system distribution (or SIS) file disguised as a security management utility. When the infected file is launched, the mobile phone’s screen displays the word “Velasco” and the worm modifies the Symbian operating system so that Cabir is started each time the phone is turned on.

Infected mobile phones scan for vulnerable phones using the phone’s Bluetooth wireless connection, then send a file, velasco.sis, that contains the worm to those phones. While the new Cabir variants do not destroy data on the phones they infect, they do block legitimate Bluetooth wireless connections and rapidly consume the phone’s battery, F-Secure said.

Both new Cabir variants have been changed so that they can spread more quickly than earlier versions of the worm. For example, unlike earlier variants of Cabir, the Cabir.H and Cabir.I can search for and find a new target if another vulnerable phone goes out of range, the company said.

“In conditions where people move around and new phones come in contact with each other, the Cabir.H and Cabir.I can spread quite rapidly,” the company said in a statement.

To be infected by Cabir, mobile phones must be running vulnerable versions of the Symbian Series 60 software and have the Bluetooth wireless feature in “discoverable” mode, making them open to new connections, F-Secure said.

F-Secure researchers are concerned by the similarity between the latest Cabir variants and the original worm code, which could mean the virus author has released the source code for the worm on the Internet.

“These new variants seem to be recompiled versions based on original Cabir source code. Which means that the Cabir source code is floating around in the underground. Which is bad news,” the company said in a statement.

Other computer viruses that have had their source code published online, including the Bagle and Phatbot worms, quickly spawned hundreds of variants, as less sophisticated virus writers and malicious hackers modified the code and released their own version of the original worm.

F-Secure has not seen a copy of the Cabir code online. However, the pace of development for Cabir has accelerated in recent weeks, which may support the contention that the code has been distributed. Seven new versions of the Cabir worm appeared since Dec. 9, after months in which no new variants were spotted. There have also been two new versions of Skulls, a Trojan horse program that affects mobile platforms since the original was discovered Nov. 19, F-Secure said.

Most antivirus companies, including F-Secure, Trend Micro Inc. and Symantec Corp., offer antivirus software for mobile phones that can detect the new versions of Cabir. However, as of Tuesday, other antivirus companies had not followed F-Secure and issued alerts about the new Cabir variants.

With only a handful of examples of malicious code yet discovered, and few reports of infections outside of the antivirus lab, the threat of infection from mobile phone viruses is still very low compared with traditional computer viruses, antivirus experts agree.